UNITED STATES ATOMIC ENERGY COMMISSION
CHEMICAL EXCHANGE AS A VERSATILE
ISOTOPE SEPARATION PROCESS
G. H. Clewett
November 7, 1950
Oak Ridge National Laboratory, Y-12 Area
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CHEMICAL EXCHANGE AS A VERSATILE ISOTOPE SEPARATION PROCESS*
By G. H. Clewett
The successful large-scale separation of the uranium isotopes by two entirely different methods
has led to an acceptance of isotope separation as an industrial process. The highly successful gaseous
diffusion process has taken its place as a routine operation. At the same time the electromagnetic
process has been demonstrating its versatility by supplying separated stable isotopes of nearly all
existing kinds for research purposes. Various studies have shown that certain separated stable
isotopes have properties of unusual value in nuclear science and technology. It is not unreasonable
to believe that a day may come when separated isotopes of many elements will be available in bulk
quantities for use in reactor construction and associated projects as well as for laboratory research.
Thus isotope-separation processes may be destined to play an even greater role in the future develop-
ment of nuclear energy than they played in the past. To bring about such a state, it will certainly be
necessary to be concerned with costs and to develop those isotope-separation processes which are
For the lighter elements, the chemical-exchange method introduced and developed by Urey is
generally accepted as being one of the most efficient. The close approach to thermodynamic equi-
librium achieved in chemical-exchange processes with resultant minimal expenditure of energy at
each stage leads to highly efficient low-cost production. 1 This is true if a suitable isotopic-exchange
reaction can be found for the particular element in question. This necessity of searching for suitable
reactions for each element prevents the chemical-exchange method from even approaching the versa-
tility of the electromagnetic process. It arises, of course, from the heterogeneity of the chemical
properties of the various elements. However, if the chemical and chemical-engineering requirements
to be met if a successful process is to emerge are examined and the fields of knowledge which are
involved are noted, it will be seen that the progress of research in these fields is such as to make the
job continually less difficult. In fact, a condition is approached where it is possible to visualize the
chemical-exchange process as having a high degree of versatility. In order to illustrate this, the
following criteria have been reviewed.
The specifications to be met by a suitable chemical-exchange system can be grouped into three
chemical requirements and three engineering requirements. Reference to specific features of one of
Urey's very well known and highly successful separation processes as these six points are listed
will serve to clarify each in turn. The enrichment of N" by exchange between NH3 and NH provides
an excellent example for this purpose. 2
CHEMICAL CRITERIA FOR A SUCCESSFUL PLANT
The three chemical criteria may be expressed as follows:
1. The element must be distributed between two phases
NH 3 (gas)-NH 4 (liquid)
*Work performed under Contract No. W-7405-eng-26.
2.' Rapid isotopic exchange must take place between the chemical forms in the two phases
N 14 H 3 + N 15 ^ — N 15 H 3 + N 14 ^
3. The equilibrium constant of the isotopic exchange reaction must be other than unity
, JNl5H 3 ] [N"H 4 + ]
[n 14 « 3 ] [n 15 h 4 + ]
1.035 at 298°K
The reason for the stipulation of the two phases is rather obvious, but the rule can be broken.
Thus, at least one successful example of a complete single-phase system is known. Bernstein and
Taylor 3 were able to obtain enrichment of carbon 13 by use of the single-phase exchange
c 13 + c 12 2 =** C 12 + C 13 2
These workers were able to move the two species countercurrently by the use of a thermal-diffusion
column. Since this is not a very efficient means of obtaining countercurrent flow, it is apparent that
a two-phase system is preferable wherein simple pumps can be used. Immiscible pairs which might
be used are shown in Table 1 .
1. Gas-liquid 4. Liquid-solid
2. Gas-solid 5. Solid-solid
Almost all the reported successful chemical-separation processes have consisted of gas- liquid
systems, and this type appears to be superior to all others. Examples of these are shown in Table 2.
(aq. compds) Reference
CN" 4, 5
No clear-cut demonstration of a gas-solid system has been reported, although preferential
adsorption of heavy-water vapor on carbon has been proposed as an isotope-separation method. 9
Something of this type might be possible, using fluidization techniques, if the solid particles could be
made small enough for rapid isotopic exchange to take place.
There appears to be no reason why a liquid-liquid system should not be satisfactory, yet only one
example has appeared — the enrichment of *>Li by exchange between lithium amalgam and an organic
solution of a lithium salt. 10
Semisuccessful liquid-solid systems have been reported, such as zeolite and ion-exchange resin
separations of the isotopes of potassium and lithium. 11 However, these were all inherently batch proc-
esses and are not amenable to development into continuous processes.
Solid-solid systems are completely unlikely.
The best possibilities are the two pairs, gas-liquid and liquid-liquid systems. Certainly, wherever
possible, it would be desirable to have a gas-liquid system. Generally, however, only the nonmetallic
elements provide suitable gaseous species, which leaves the necessity of considering a liquid-liquid
system in most cases. It would appear then that the liquid-liquid type might be regarded as the most
At this point it may be worth while to examine the third criterion, i.e., that the equilibrium con-
stant for the isotopic-exchange reaction must be other than unity. Urey 12 ' 13 and others' 4 have shown
how these constants may be calculated, and some degree of qualitative familiarity with the method is
quite helpful in understanding which chemical equilibria lead to the largest separation factors.
Bigeleisen and Mayer 14 devised two relatively simple formulas for calculating equilibrium constants.
They point out several simplifying cancellations which take place in calculating ratios of partition
functions for isotopic equilibria. Since the translational and rotational partition functions are classi-
cal at room temperature in all cases except for hydrogen, the concern is solely with the vibrational
partition function. The most general of the two formulas mentioned above is*
|-f = 1 ♦liGd'i) Ai^i
where G = 2~*V-1
" = £t"
Ai/ = change in v upon isotopic substitution.
The second formula is for polyatomic symmetrical molecules containing isotopes of a heavy
zrf = 1
U, = -r=; and V, is the totally symmetric, or "breathing frequency"
AM = difference in mass between the isotopes of the heavy element
m = atomic weight of atoms bonded to the central heavy element
M = mass of heavy element
n = number of atoms bonded to the central heavy element
Now, let us examine the ratio of partition functions for some isotopic chlorine molecules.
ci 37 o
ci 35 c.
ci 35 o-
*Bigeleisen and Mayer carry the simplification further and obtain
s y A (Ui )2
ir f = 1+ A—ir-
as a very rough approximation.
Thus the equilibrium constants for the following isotopic-exchange reactions would be
1/2 ClJ 7 + Cl35o 2 ^ 1/2 elf + C1 37 2 K = ijjgg = ^
l/2Clf + Cl3 5 03^1/2Cl 35 + Cl3V - K = ^ = 1.040
1/2 Cl" + C1 35 0~ ^ 1/2 CI 35 ♦ C1 37 0~ K = ^jj|g = 1.077
Urey 13 has pointed out that these are progressively larger with added oxygens because of the
increased number of vibrational degrees of freedom and the approximate constancy of the vibrational
frequencies of all the oxygen compounds of chlorine. That this is not always so is demonstrated by the
examples chosen by Bigeleisen and Mayer 14
28 r . o;30 r --^o;30iT c;28v-.- ,r 1-111
Si 28 F 4 + Si 30 F6~=iSi 30 F 4 + Si 28 Fg
K = no9^- 002
Sn 12 C l 4 + Sn" 8 Ci: =* Sn 118 Cl 4 + Sn^d! K = |^|M = 1.00025
D ^ D l.UUZOl
Here a considerable cancellation effect is observed because the bonds have become weaker in
the octahedral ion than in the tetrahedral ion, and the total restoring force is approximately unchanged.
Other examples are available, but these allow a qualitative rule to be formulated for choosing suitable
isotopic-exchange reactions. In order to obtain a large separation effect, it is evident that the element
in question should exist in one phase unbonded or with only a few weak bonds, and in the other phase it
should be bonded with as many bonds as possible, and these bonds should be strong.
When the second chemical requirement of rapid isotopic exchange is examined, some degree of
incompatibility is found between the various criteria. The species desired in one phase with many
strong bonds is not likely to undergo rapid isotopic exchange with the free element, the ion, or a
loosely bonded molecule. If the element is a metal, then one of the first choices of a pair of dissimilar
forms is the metal ion in aqueous solution in equilibrium with a chelate complex or other complex of
the metal in an immiscible organic solvent. Such coordination entities supply the many strong bonds
which lead to a large separative effect, but these bonds are usually considered to be covalent, with
resultant negligible exchange. However, if the presence or absence of exchange is accepted as a
criterion of bond type, then it must be concluded that covalent bonds are not universally present in such
coordination entities since there are numerous examples in which very rapid isotopic exchange takes
place. The status of chemical knowledge of the structure of coordination compounds has been briefly
summarized recently by Fernelius, 15 and his list of exchange reactions involving coordination com-
pounds contains some which are described as very fast.
Since the success or failure of the Szilard-Chalmers method of producing concentrated radioactive
species is intimately related to ease of exchange, a study of some compounds which are not amenable
to this treatment is helpful. Fernelius 15 lists a few compounds which apparently exhibit exchange,
since the Szilard-Chalmers method was unsuccessful with each.
Mg ++ with Mg(8-hydroxyquinolinate)2
(aqueous E + OH)
Zn (pyridine)2 (OAc)2 with
Zn benzoylacetonate ammoniate
Zn nicotinyl acetonate
Zn (pyridine) 2 (SCN) 2
CN~ with Hg(CN) 4 (pH 10)
Mn ++ with Mn(C 2 4 )3
Ir0n -4 -3
Fe(CN) 6 with Fe(CN) 6
CN~ with Ni(CN) 4
Ni ++ with Ni (salicylaldehyde)3
Ni ++ with Ni (ethylene diamine^
Ni with Ni (salicylaldoximo)3
Ni with Ni (salicylaldimino)„
Br" with PtBr 4 ~ and PrBrg"
Br in cis-[Pt(NH3) 2 Br 2 l with [PtBr 4 ]~~
Br in [PtBrg]"" with [PtBr 4 ]"
Cu ++ with Cu (acetylacetonate) 2 in CHCI3
Cu + ^ with Cu derivations of salicylaldehyde
and related compounds
Table 4 — Compounds Not Amenable to Szilard-Chalmers Effect
Zn (acetylacetonate) 2
V 2 C»3 (8-hydroxyquinoline) 4
U0 2 (benzoylacetonate)2
Mn (benzoylacetonate) 2
There seems to be no marked inconsistency between the conclusions from these experiments
regarding bond type and the conclusions from other studies such as magnetic susceptibility measure-
ments and experiments which differentiate between the planar configuration of covalent dsp^ bonds and
the close-packed or tetrahedral arrangement of ions. Some apparent anomalies can be explained by a
difference in bond type between the solid compound and a solution of the compound. In other cases
certain solvents seem to convert part or all of the covalent molecules into complexes of ionic bonds,
thus allowing exchange to take place. The point to be noted is that significant advances are being made
in systematizing the knowledge of the structure of coordination compounds and, specifically, the condi-
tions required for isotopic exchange.
The application of this widening knowledge of complex compounds to isotopic equilibria in a
quantitative way is some distance away as yet. It has been shown that even an approximate calculation
of these equilibrium constants requires a knowledge of the vibrational frequencies arising from bonding
of the element in question, and in most cases the shift in frequency upon isotopic substitution must also
be known. If these frequencies are unknown and estimates are to be made, then it is, of course, very
important to choose the proper atomic model. It will be remembered that the aim is to equilibrate some
complex compound or ion of the isotopic element with some simple compound or ion. It would appear
that the information about vibrational frequencies would not be difficult to obtain for the simple species
in any case. However, where the element in question is a metal, the simple species will most likely
be the simple positive ion. It appears that the water of hydration of such metal ions profoundly affects
the vibrational partition function, and yet there is no measure of the frequencies of vibration of the
metal-oxygen bonds in such hydrates, if, indeed, these frequencies have any physical meaning.
It appears that they are completely absent in raman and infrared spectra. Bigeleisen 31 offers the
explanation that perhaps the rate of exchange of water molecules or oxygen atoms about such a metal
ion is of the same order of magnitude as the vibrational frequencies which might otherwise be observed.
At any rate, it appears that the presence of water of hydration on these simple metal ions intro-
duces an unknown factor in the problem of calculating isotopic equilibria. On the other hand, it may be
of some academic interest that an experimental determination of the isotopic equilibrium factor for
suitable individual cases provides an indirect method of learning something about the strength of these
ion dipole bonds. Suppose, for example, the equilibrium
X A +y AC 2 Y A +X AC 2
where A represents a divalent metallic ion, AC 2 represents an extractable complex, and the super-
scripts represent two different isotopes of the element A. Suppose further that all vibrational fre-
quencies are known for AC2 and that the equilibrium constant for this exchange reaction is known. It
can be seen that through the use of the formulas of Bigeleisen and Mayer, the usual calculation can
be reversed, and a numerical result can be obtained for a hypothetical symmetrical vibrational fre-
quency for the hydrated ion. The accuracy of such a calculation is entirely unknown.
It is apparent that the position as regards the versatility of quantitative calculations of isotopic-
exchange constants is not a strong one. However, the methods of experimentally measuring these
constants are straightforward even though rather tedious, so that the chemical-exchange method of
separating isotopes is very little less versatile because of this difficulty in calculating the single-
From a chemistry standpoint, if a particular system satisfies the three rules, (a) distribution
between two phases, (b) rapid isotopic exchange, (c) equilibrium constant other than unity, then it is
ready for engineering development.
ENGINEERING CRITERIA FOR A SUCCESSFUL PLANT
Consider the development problems to be met in applying any exchange system. For the success-
ful application of an isotopic-equilibrium reaction, certain conditions must be met which are more of
the nature of chemical-engineering principles. There are various ways of expressing these require-
ments, but for the present purposes they may be presented as follows.
1. The two immiscible phases of the chemical-exchange system must be efficiently contacted as
2. The apparatus must have low holdup and high throughput.
3. Means for achieving total reflux at the top and bottom of the plant must be provided.
When the first two points are considered together, the various means available for efficiently
contacting two phases are discussed briefly below. This is where the real advantage of having a
system which involves one gas and one liquid phase becomes apparent. That is, a simple packed
column suitable for distillation provides a means of contacting these two phases efficiently with very
short stage height. Furthermore, such distillation columns have been studied extensively in all sizes
and pose no great problem in equipment design. However, as noted earlier, very often it will be
impossible to devise a system having one gas phase, and probably liquid-liquid systems must be dealt
with in most cases. When two liquids are contacted in an ordinary packed column, the stage heights
are usually much greater than those observed with gas-liquid systems. That this should be of concern
in no small degree will be manifested if a simple mathematical function in plant design is examined,
namely, the time to reach equilibrium.
In most distillation processes the single-stage factors are so large that the entire plant reaches a
steady state or equilibrium condition almost as soon as it reaches thermal equilibrium. With an
isotope -separation plant, the factor is usually so small and the plant is so long in terms of number of
stages that there is a considerable time lag in starting up while the total plant production must go into
building up the inventory of enriched material throughout the plant before any product may be with-
drawn. The time to reach such a steady state is called the "time to reach equilibrium." Since it is
dependent upon the rate of production as well as the size of the plant, it provides a simple measure of
plant efficiency. The function is of the following form
T = -^ *(No,N)
where h = processing time per stage
E = a-1
4>(No,N) = function of No and N, which are mole ratio of desired isotope to undesired
in product and feed, respectively
The product and feed level are usually fixed. The single-stage separation factor is also fixed for
any one system, but since it enters as a square term here, it simply points up further the importance
of having a large separation factor. The main concern then is with the processing time per stage. If
the rate of exchange of the isotopic reaction is very rapid, the processing time per stage becomes a
function of the ratio of holdup per stage to throughput or interstage flow. Thus, in engineering an
isotope-separation plant it is very important to achieve a short-stage height with high throughput.
These same features are highly desirable in any plant using a liquid-liquid process, although not
so critical in most, and continual progress is being made in devising new and improved equipment for
this purpose. Besides various types of packed columns, there are spinner columns, 32 centrifugal
separators, 33 as well as simple mixer settlers. Most engineers are familiar with these and many other
contacting devices. The development of any design which achieves a significant decrease in the stage
height or the ratio of holdup to flow in each stage can mean the promotion to complete feasibility of
many otherwise borderline or impractical processes.
The final criterion to be met, of achieving reflux at top and bottom of the plant, is not entirely
an engineering problem. First of all, it is a chemical problem of converting the element in question
from one chemical form to another. Here again, Urey's process for enriching N" provides an
excellent example. The nitrogen, present as ammonium ion in the liquid phase, is converted to
ammonia gas by treatment with caustic. The ammonia gas then travels back up the column to the top
where it can then be reconverted to ammonium ion with acid. The net result of these chemical proc-
esses is the neutralization of an acid with a base to produce a salt. Benedict 1 has pointed out that
this net process and the acid and caustic chemicals used correspond to the power costs or energy
costs in other types of separation plants. Because of the large reflux ratio, or ratio of interstage flow
to product flow required in an isotope-separation plant, large quantities of chemicals are required
usually in this reflux operation. Dependent upon the chemistry involved, equipment must be devised
for each different process to accomplish this reflux. It must be done quantitatively with a minimum of
Since the operating principle of each reflux mechanism is so dependent upon the chemistry of the
system itself, very few generalizations are possible. Sometimes the chemistry of a particular system
is such as to make it difficult if not impossible to attain this satisfactorily. For these cases it is
important to note that a means of obviating the need for reflux has been proposed which is based upon
the temperature coefficient of the separation factor. 34 The equilibrium constant for an isotopic-
exchange reaction usually varies with temperature in some such manner as shown in Fig. 3. By
suitable selection of Tj and T£, two towers can be used, a hot tower and a cold tower, operating with
two different separation factors. Figure 4 shows how these towers are then connected. If the large
factor of the cold tower favors the incoming stream at the top, then the smaller factor of the hot tower
favors this same stream somewhat less as it emerges from the bottom of the hot tower. Thus, we have
a build-up of desirable component between the two towers as shown in this diagram. The hot tower acts
somewhat as a reflux mechanism since it prevents the exit from the system of desirable component
by means of its smaller separation factor. Since this eliminates the need for large amounts of chemi-
cals in the ordinary type reflux device, it might be concluded that this dual-temperature method would
have somewhat of an advantage. However, this ease of reflux is counterbalanced more or less by the
fact that a much larger plant must be built and operated because of the much lower net factor using
the dual-temperature process which leads to a larger number of stages. Expressed graphically on a
McCabe-Thiele diagram, Fig. 5, it is easy to see that many more stages are required.
These then are the various criteria for successful application of chemical exchange to the
separation of isotopes. It is believed that the advances now being made in the fields of knowledge
mentioned above are such as to materially extend the applicability of this method.
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9. D. F. Stedman, National Research Council, Canada, unclassified report C50-49S.
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13. H. C. Urey, J. Chem. Soc, 562 (1947).
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15. W. C. Fernelius, Record Chem. Progress, Winter 1950.
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31. Bigeleisen, verbal discussion.
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33. Podbielniak Industrial Research and Engineering Laboratories, Chicago.
34. Dual Temperature Process, MDDC-891.
NH 4 N0 3 —
^ * NH 3
Fig. l—N 1 ^ (gas) + N 1 ^ (sol.) ^1%^ (gas) +
N 1 !^ (sol.) [h. G. Thode and H. C. Urey, J. Chem.
Phys., 7: 3^ (1939)].
co 2 +co -
Fig. 2— C 12 2 (gas) + C^O (gas) ^ C^Og (gas) +
C 12 (gas) [r. B. Bernstein and T. I. Taylor, J.
Chem. Phys., l6: 903 (19^8)].
Fig. 3 - -Variation of the equilibrium constant for an isotopic-exchange
reaction with temperature.
Fig. k — Diagram of hot and cold towers connected.
Fig. 5 — McCabe-Thiele diagram.
END OF DOCUMENT
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